With the rapid industrial and manufacturing development in the global scale, use of fossil fuels, such as petroleum, has been increased, and international oil prices have been steadily soaring. Accordingly, stable production and supply of crude oil has become an increasingly important issue.
For this reason, petty deep sea oil fields, which have been neglected until recently for their technical difficulties of drilling and lack of economic feasibility, have begun to receive attention, and ships having drilling equipment fitted for oil field development have been developed in step with the development of resource development technologies.
For the conventional sea drilling equipment, a rig ship or a fixed-type platform have been commonly used, which can be moved by another tugboat and carries out its drilling operation while being fixed at a location in the sea by mooring equipment.
Developed and used more recently for deep sea drilling is a drillship, which is closer to an ordinary vessel with the state-of-the-art drilling equipment and can sail with its own locomotive power. Such drillship needs to be designed to have optimal sailing capacities as well as drilling capabilities because it needs to frequently move its location for the development of petty oil fields.
The drillship is installed with a large opening (referred to as “moonpool” hereinafter) for lowering a drilling pipe. Although this structure is indispensably essential for the use of the drillship, this structure is very disadvantageous in terms of the sailing speed.
In other words, due to periodic oscillation of the water surface inside the moonpool caused by the relative movement between the seawater flowed in and out of the moonpool and the seawater outside the hull, and the resulting movement of the hull, resistance is increased during the sailing of the drillship, resulting in the reduced speed and more fuel consumption. It has been observed that this resistance is increased by as much as 50%.
To date, designed and utilized for the purpose of reducing such increase of resistance have been affixture affixed inside the moonpool, affixture on the bottom of the ship around the moonpool, movable opening/closing devices inside the moonpool, etc. However, the affixture inside the moonpool has a complicated structure compared to its effect, and the movable opening/closing device is not widely used because its cost for installation and maintenance is very high.
Contrived to solve the above problems, the present invention provides a moonpool and a drillship having the moonpool that can reduce the resonance and resistance caused by the vertical movement of the seawater inside the moonpool while the drillship is sailing.
The present invention also provides a moonpool and a drillship having the moonpool that can reduce the amplitude of a sloshing movement and the resistance caused by the sloshing movement of the seawater inside the moonpool while the drillship is sailing.